1. Field of the Invention
The present invention is related to a radiation image processing apparatus that removes periodic patterns from radiation images.
2. Description of the Related Art
Various types of radiation image detectors that generate charges by being irradiated with radiation which has passed through subjects and record radiation images of the subjects by accumulating the charges have been proposed and are in actual use in the field of medicine and the like. For example, there are radiation detectors that utilize amorphous selenium, which generates electrical charges when irradiated by radiation. In radiation image processing apparatuses, scattered radiation removing means (grids), constituted by a plurality of lead plates which are arranged parallel to each other, are provided between radiation sources that emit radiation and the radiation image detectors. This configuration enables scattered components of radiation to be removed by the scattered radiation removing means. When radiation images are obtained via the aforementioned scattered radiation removing means, periodic stripes are generated within the radiation images due to the scattered radiation removing means. Various proposals have been made to prevent deterioration of image quality due to the periodic stripes.
Japanese Unexamined Patent Publication No. 2002-330344 proposes deriving spatial frequency components of periodic patterns, which are determined by the lattice density of a scattered radiation removing means, then removing periodic patterns by administering a filtering process that removes frequency components having frequencies greater than or equal to the spatial frequency from radiation images. The invention disclosed in this document presumes that periodic patterns exist in the high frequency side, and that images of subjects are not present in frequency ranges greater than or equal to the spatial frequency of the scattered radiation removing means. U.S. Pat. No. 6,075,840 proposes to provide the plates of a scattered radiation removing means such that they are inclined at a predetermined angle with respect to a radiation detector. Further, U.S. Pat. No. 7,039,151 proposes to reciprocally move a scattered radiation removing means so as to prevent the generation of periodic stripes, which are caused by still scattered radiation removing means.
Recently, it is desired for lattice densities of scattered radiation removing means to be decreased, so as to improve the transmissivity of radiation. Here, in the case that the lattice density of the scattered radiation removing means of Japanese Unexamined Patent Publication No. 2002-330344 is decreased, the periodic patterns which are caused due to the scattered radiation removing means will be generated at the low frequency component side of the spatial frequency components. In addition, if the scattered radiation removing means is provided in an inclined manner or reciprocally moved as in U.S. Pat. Nos. 6,075,840 and 7,039,151, decreased lattice densities will not eliminate periodic patterns, but rather cause them to appear as low frequency components.
Meanwhile, image data that represent tissue structures are present as lower frequency components. In cases that the spatial frequency components of periodic patterns appear as low frequency components, there are cases that image components of subjects are present as higher frequency components than periodic patterns. Accordingly, there is a problem that frequency components that represent tissue structures will be removed as well, if all frequency components which are greater than or equal to the spatial frequency of the periodic pattern are removed, as in Japanese Unexamined Patent Publication No. 2002-330344.
Purposefully providing scattered radiation removing means at desired angles in order to avoid inclinations with respect to the main direction or a sub direction of a radiation detector and to prevent moiré patterns with the radiation detector from being generated, may be considered. These grids have angular components in addition to frequency components. Therefore, if these components are all removed, there is also a possibility that frequency components that represent tissue structures will be removed.